It has hitherto been known that Serratia marcescens has aspartase activity, and a method of enzymatic production of L-aspartic acid from ammonium fumarate (or fumaric acid and an ammonium salt) by using such a microorganism has been also known in the prior art (Japanese Patent Publication Nos. 12553/1979 and 18867/1982). However, aspartase activity of Serratia marcescens used in such a conventional method is not so high as it is sufficient for the industrial production of L-aspartic acid.
Under the circumstances, in order to establish an industrially advantageous method for producing L-aspartic acid, the present inventors have intensively studied. As the result, the present inventors have succeeded in the isolation of a microbial strain having very high aspartase activity in comparison with that of its parent strain by a so-called molecular cloning, i.e., by isolating a chromosome fragment of a microorganism of the genus Serratia which specifies aspartase activity, joining it to a plasmid and then introducing the plasmid into a microorganism of the genus Serratia, and found that L-aspartic acid can be very efficiently produced by utilizing the microorganism thus isolated.